In the present state of the art, low friction under lubricated tests is generally attainable through the formation of CH3-terminated tribofilms or in the presence of solid lamellar compounds such as MoS2 or boric acid, for example. However, almost typical friction coefficients obtained are not lower than 0.04 and lower than 0.1, and therefore values lower than 0.04 have hardly been reported so far.
Global environmental problems such as global warming and ozone layer destruction are coming to the fore. As it has been said that the global warming is significantly affected by CO2 emission, the reduction of CO2 emission, notably the setting of CO2 emission standards, has become a big concern to each country. One of the challenges to reduce CO2 emission is to lower an energy loss due to friction loss of machines, facilities and the like, particularly to improve vehicle fuel efficiency or fuel economy that depends on the performance of engine sliding members and a lubricating oil applied thereto. There are the following approaches to improve the vehicle fuel efficiency: (1) lowering the viscosity of the lubricating oil, thereby reducing viscous resistance in hydrodynamic lubrication regions and agitation resistance in the engine; and (2) adding a suitable friction modifier and other additives into the lubricating oil so as to reduce friction losses under the conditions of mixed lubrication and boundary lubrication.
A patent literature 1 discloses a low-friction sliding member with a base material having a surface and a hard carbon thin film formed on at least a part of the surface of the base material, wherein a tribofilm having at least one functional group selected from the group consisting of an ether linkage, an oxido and a hydroxyl group is formed on the hard carbon thin film when the hard carbon thin film is in slidable contact with an opposite member in the presence of an organic oxygen-containing compound.
The patent literature 1: European Patent No. 1510594A specification